The overall objective of this proposal is to define and characterize the molecular and cellular mechanisms that regulate the plasminogen system. As the primary fibrinolytic enzyme, plasmin is essential for hemostasis and the maintenance of vascular patency. In addition, plasmin, with its broad substrate recognition, has been implicated in numerous physiologic and pathophysiologic processes involving cell migration, including tissue remodeling, angiogenesis, tumor cell invasion and microorganism infection. Substantial evidence suggests that plasminogen activation in these various processes occurs upon and is restricted to cell surfaces. Plasminogen binds to cellular receptors, where it is preferentially activated to plasmin and protected from inhibitors. This proposal seeks to define the mechanisms which regulate the binding and activation of plasminogen on cell surfaces as well as to critically test the role of plasminogen in cell migration in vivo. The hypothesis of this application is that specific control mechanisms modulate the function of the plasminogen system on cell surfaces, thereby regulating proteolysis within the microenvironment and controlling the contribution of the plasminogen system to cell migration. Thus, the plasminogen system is subject to precise control mechanisms in the same way as the activities of the coagulation, protein C and complement systems are regulated. Four specific aims are proposed to test this hypothesis: 1) The role of cell-surface proteolysis in upregulating plasminogen receptor expression and basic carboxypeptidases in downregulating these receptors will be assessed as a means of defining a previously unrecognized and potentially very important regulatory mechanism. 2) The plasminogen receptor(s) that is critical to the acceleration of plasminogen activation on cell surfaces will be identified and characterized. 3) The role of cell adhesion in modulating plasminogen receptor expression will be assessed using both model and physiologic cell types. 4) The contribution of the plasminogen system to selected cell migratory responses will be directly evaluated by comparing these responses in wild-type and plasminogen-deficient mice. In addition, experiments will determine whether the carboxypeptidase(s) also exert control of fibrinolysis in vitro and in vivo. On an overall basis, the proposed studies should extend and provide new insights into the complex interface between cells and the plasminogen system in vitro and in vivo. These studies also should serve as models to define the mechanisms which control proteinases and their activities at cell surfaces.